foundations
Persistence of vision
Stop motion
Definition
Persistence of vision is the historical theory that the eye briefly retains each image on the retina after it disappears, and that this retinal afterglow is what makes a rapid sequence of still frames read as continuous motion. Vision science has shown this to be false as an account of motion perception -- a retinal afterimage is static and would blend into a smear rather than directed movement -- though the phrase survives informally as a loose stand-in for "why film and animation appear to move." The mechanisms that actually do that work are apparent motion (how the visual system links successive still positions into one moving object) and critical flicker fusion (a separate threshold governing when an interrupted light source reads as continuous rather than pulsing).
Overview
The theory dates to nineteenth-century optical-toy science (the phenakistoscope and zoetrope era) and was carried into early film theory as a ready explanation for why a strip of still photographs, run past the eye fast enough, looks like continuous movement: the retina supposedly holds each frame just long enough to overlap with the next. Joseph and Barbara Anderson challenged that account directly in the Journal of the University Film Association (1978) and again, more fully, in "The Myth of Persistence of Vision Revisited" (Journal of Film and Video, Vol. 45, No. 1, Spring 1993): an afterimage is the wrong kind of evidence to explain apparent motion, because an afterimage is a static, fading impression, and stacking static impressions on top of each other would produce a blurred double exposure, not a moving figure. Writing for animationstudies 2.0 in 2024, animator and educator Philippe Vaucher restates the same point for an animation-specific audience, summarising the objection in one line: "afterimages are static and, if blended, produce a collage effect rather than smooth motion."
What replaced the myth is two separate, better-evidenced mechanisms. The first is apparent motion, and it comes in two registers. Short-range (first-order) motion links positions that are close together in space and time -- the condition ordinary live-action footage satisfies by default, since a real object's position barely changes from one frame to the next -- and it reads as smooth, naturalistic movement. Long-range (second-order) motion covers the case where the gap between successive positions is larger; the visual system still links the positions into one moving object, but Vaucher notes the result draws more on memory and attention than on the low-level motion system, and looks correspondingly more mechanical. The second mechanism is unrelated to motion perception at all: critical flicker fusion (CFF) governs whether a light source that is switching on and off -- a projector shutter, a display refresh, a held stop-motion exposure -- reads as continuous illumination or as visible pulsing. A 2021 narrative review in Medicina puts ordinary CFF at roughly 50 to 90 Hz for most viewers, while noting it shifts with luminance, retinal location, age and other individual factors -- which is why the same footage can flicker for one viewer and look steady to another.
Stop motion sits directly on the boundary the first mechanism describes, because the technique's whole premise is manufacturing large frame-to-frame position changes on purpose. Animating on twos (see on-twos) roughly doubles the distance a puppet appears to travel between exposed frames compared with animating on ones -- exactly the variable that pushes a shot from the short-range system toward the long-range one -- and the craft treats that shift as a visible style choice, not a defect. Fantastic Mr. Fox's puppet maker Peter Saunders has described Wes Anderson choosing twos specifically so the movement kept a slightly stepped character, a deliberate long-range-motion look rather than pursued smoothness. Cinematographer Frank Passingham's account of Guillermo del Toro's Pinocchio (guillermo-del-toros-pinocchio) shows the same boundary crossed the other way: fast lateral tracking shots animated on twos produced objectionable strobing at 4K, so the production switched those specific shots to ones, pulling the frame-to-frame gap back under the threshold where it reads as motion rather than flicker. See also flicker-control-stop-motion for the illumination-side half of the same boundary.